1. Field of the Invention
The present invention relates to a reversible thermosensitive recording material, and more particularly a reversible thermosensitive recording material able to repeatedly record and erase images by reversibly changing its transparency or color, as well as to methods of using such a thermosensitive recording material.
2. Disscussion of the Related Art
Reversible thermosensitive recording materials have lately been a subject of interest due to their advantageous ability to reversibly record an image and erase the image when the image is not necessary. For example, Japanese Laid-Open Patent Applications No. 54-119377 and 55-154198 have disclosed reversible thermosensitive recording materials in which a low-molecular-weight organic material such as a higher fatty acid is dispersed in a resin such as a vinyl chloride-vinyl acetate copolymer having a relatively low glass transition temperature of from 50 to 80.degree. C. The reversible thermosensitive recording material records an image using a change between a transparent state and an opaque state thereof; however, it has a drawback that the temperature range in which the reversible thermosensitive recording material is in a transparent state is narrow, i.e., from 2 to 4.degree. C., and therefore temperature control for recording the image is difficult.
In attempting to broaden the temperature range, reversible thermosensitive recording materials have been disclosed in which a low-molecular-weight organic material having a relatively wide temperature range is employed in a recording layer or plural low-molecular-weight organic materials are used in a recording layer (Japanese Laid-Open Patent Applications No. 63-39378, 2-1363, 3-2089 and 5-294066). These reversible thermosensitive recording materials have good image erasability (an ability to erase an opaque image so as to be in a transparent state) when an opaque image is erased by heating with heating media having a relatively long heating time such as heat rollers; however, they have poor image erasability when heated with heating media having a relatively short heating time such as thermal printheads.
In attempting to solve this problem, i.e., poor image erasability when heated with thermal printheads, a reversible thermosensitive recording material has been disclosed which has an appropriate transparent state starting temperature changing rate (the changing rate of the temperature from which the reversible thermosensitive recording material starts to be transparent) of the reversible thermosensitive recording material, or an appropriate transparency changing rate or an appropriate thickness changing rate of the recording layer (International Laid-Open Patent Application No. WO95/20491). The reversible thermosensitive recording material has good image erasability when heated with thermal printheads. Since reversible thermosensitive recording materials are preserved in various environmental conditions because of having the ability of repeatedly recording and erasing images, when even such a reversible thermosensitive recording material having good image erasability with thermal printheads is preserved in high temperature environments after an opaque image is recorded therein, a problem occurs that the opaque image cannot clearly be erased (namely, the erased thermosensitive recording material has lower transparency than ever) by a relatively short time (i.e., a few msec) image erasure using a thermal printhead. Therefore, contrast of an image decreases, and thereby readability of the image deteriorates. This is a new problem and a solution has not heretofore been proposed.
Because of these reasons, a need exists for a reversible thermosensitive recording material which has good contrast of images, i.e., good readability of images, even after the reversible thermosensitive recording material is preserved in an opaque image state (a second color state) under high temperature environments.